EFFECT OF IONIC INTERACTION IN MIGRATION TESTING OF CHLORIDE DIFFUSIVITY IN CONCRETE

The actual drift velocity of chloride ions in concrete measured by migration testing in concentrated NaCl solutions not only depends on the applied electric field but also on the ionic interaction. A net velocity V-d is calculated considering the relaxation and electrophoretic effects of the ionic cloud around an ion described by the Debye-Huckel theory. V-d is dependent on ionic concentration, ionic mobility, applied electric field, and other parameters. A correction factor, beta(0), which is the ratio of the ideal to the actual drift velocity of ions, is calculated for NaCl solutions up to 0.5 M. Higher salt concentrations do not permit correction since the Debye-Huckel theory is invalid when the thickness of ionic cloud is comparable to the average hydrated ionic radius. The correction factor permits calculation of the actual chloride diffusion coefficient during testing by introducing a concentration related retardation in ion migration velocity of up to 41% for a 0.5 M NaCl solution at 20 degrees C.